MEXICO CITY (AP) — An explosion at the main headquarters of Mexico's state-owned oil company in the capital killed 14 people and injured 100 on Thursday as it heavily damaged three floors of a building, sending hundreds into the streets and a large plume of smoke over the skyline.

Another 30 people remained trapped in the debris late Thursday, according to the Interior Ministry, as teams of military with rescue dogs showed up to extract the victims. The blast occurred in the basement of an administrative building next to the iconic, 51-story tower of Petroleos Mexicanos, or Pemex, one of the tallest buildings in Mexico City.

"It was an explosion, a shock, the lights went out and suddenly there was a lot of debris," employee Cristian Obele told Milenio television, adding that he had been injured in the leg. "Co-workers helped us get out of the building."

Ana Vargas Palacio was distraught as she searched for her missing husband, Daniel Garcia Garcia, 36, who works where the explosion occurred at about 3:45 p.m. local time, when many workers were just returning from lunch. She said she last talked to him a couple hours earlier.

"I called his phone many times, but a young man answered and told me he found the phone in the debris," Vargas said. The two have an 11-year-old daughter. His mother, Gloria Garcia Castaneda, collapsed on a friend's arm, crying "My son. My son."

There was no immediate cause given for the explosion in a busy commercial and residential area. Pemex first said it had evacuated the building because of a problem with the electrical system. The company later tweeted that experts from the Attorney General's Office were analyzing the explosion and any reports of a cause were speculation.

The tower, where several thousand people work, was evacuated following the blast but not damaged, according to Gabriela Espinoza, 50, a Pemex secretary for 29 years who was on the second floor when the explosion next door occurred.

"There was a very loud roar. It was very ugly," she said.

The explosion occurred in the basement of a 13-story building across from the tower in five-building the Pemex complex, according to Emporis, which collects construction data worldwide.

Espinoza's co-worker, Tomas Rivera, 32, worked on the ground floor and was knocked to floor, fracturing his wrist and jaw.

A reporter at the scene saw rescue workers trying to free several workers trapped. Television images showed people being evacuated by office chairs, and gurneys. Most of them had injuries likely caused by falling debris.

"We were talking and all of sudden we heard an explosion with white smoke and glass falling from the windows," said Maria Concepcion Andrade, 42, who lives on the block of Pemex building. "People started running from the building covered in dust. A lot of pieces were flying."

Police landed four rescue helicopters to remove the dead or injured. About a dozen tow trucks were furiously moving cars to make more landing room for the helicopters.

Streets surrounding the building were closed as evacuees wandered around, and rescue crews loaded the injured into ambulances. The injured were taken to Pemex's hospital in Azcapotzalco and the Red Cross hospital in Polanco, where relatives huddled in the waiting room for news of their loved ones. Some walked out of meetings with the hospital social worker joyous, while others came out crying.

"I profoundly lament the death of our fellow workers at Pemex. My condolences to their families," President Enrique Pena Nieto said via his Twitter account.

Shortly before the explosion, Operations Director Carlos Murrieta reported via Twitter that the company had reduced its accident rate in recent years. Most Pemex accidents have occurred at pipeline and refinery installations.

A fire at a pipeline metering center in northeast Mexico near the Texas border killed 30 workers in September, the largest-single toll in at least a decade for the company.